A hybrid circuit is used to transmit and receive signals simultaneously along the same medium. For example, twisted-pair cable is used in the public telephone network to carry signals in both directions between two people talking. A single pair of wires carries both signals. Similarly, in Ethernet transmission over copper wires, each of four pairs of wire in a Category 5 cable is used to carry signals in both directions simultaneously.
A hybrid circuit is necessary because the transmitter and the receiver can not simply be tied together at each end of the cable. If this were done, the transmit signal would severely interfere with the received signal, particularly if the receive signal has been significantly attenuated by the cable. Thus, some means must be used to cancel the transmit signal at the receiver input so that just the receive signal can be recovered, while still allowing the transmit signal to be applied to the cable.
Conventional hybrid circuits can be passive, constructed of various coils arranged so as to achieve the necessary cancellation of the transmit signal. Another type of conventional hybrid circuit employs operational amplifiers as disclosed in U.S. Pat. No. 5,479,504, by Nakano et al., herein incorporated by reference in its entirety.
The hybrid circuit disclosed in Nakano et al. is a voltage mode device in that it utilizes a voltage output for the transmitter with series termination resistors to terminate the medium as well as to isolate the transmit circuit from the receive circuit. Such a topology relies on low output impedance voltage drivers so that the receive signal is significantly attenuated at the transmitter output. In such a circuit the transmit signal, free from the receive signal, is available so that it can be subtracted from the combined transmit and receive signal that appears at the interface to the medium.
One drawback of the above referenced hybrid circuit is that it is not ideal for implementation in mass produced high speed communication circuits. As such, a new circuit topology capable of use in integrated communication circuits transmitting up to 1 Gigabit per second or faster is needed.
Further, it would be desirable if such a topology allowed a controlled, stable impedance (termination) to a transmission medium. Also, it would be desirable if some implementations allowed a differential receive signal coming from the medium to pass with minimal loss to a single ended node VRX. Moreover, it would be desirable if the transmit signal were allowed to pass with low loss to the medium. In addition, it would be desirable to efficiently cancel the transmit signal from the recovered receive signal.
At least one embodiment of the present invention utilizes, a current mode differential active hybrid circuit having an H-bridge output driver with a first output resistor coupled across a transmission medium output. Drive circuitry is coupled to the output H-bridge and constructed to supply data modulated signals to the output H-bridge. A second H-bridge is coupled to the drive circuitry in parallel with the output H-bridge to allow a second output across a second output resistor.